The present invention generally relates to fasteners and threaded workpieces, and more particularly to affixing an internally threaded insert within a workpiece so that a threaded fastener may be made up to the workpiece utilizing the threads of the insert.
For a variety of reasons it is desirable to dispose a self-tapping sleeve within a workpiece. Most, but not all of the time, the self-tapping sleeve will have internal threads and will be utilized for thread replacement. For example, if pre-existing threads of the workpiece are damaged, the damaged threads may be replaced with the threads of the insert. One type of internally-threaded insert is self-tapping, such that the insert may be driven into a bore of the workpiece, cutting threads in the bore as the insert is driven. The self-tapping inserts have both internal threads for receiving a fastener and external threads. A first group of external threads cuts new threads in the bore, and a second group of external threads makes up into the new threads, thereby advancing and securing the self-tapping insert within the bore and thus providing new threads within the workpiece.
The most common use of self-tapping inserts is to provide replacement threads where the original threads of the workpiece have become damaged, and to stabilize the matrix material to prevent cracks from continuing or propagating. When the original threads become damaged, they can sometimes be repaired by chasing the damaged thread with a tap to restore the original thread shape. However, if the original thread shape cannot be restored by this measure, the thread must be replaced. One means of replacing the threads is to bore the hole to a larger diameter than the original thread diameter and to rethread the hole. However, a disadvantage of this procedure is that it requires a change in the fastener size from the original. If the equipment utilizes multiple fasteners of the original size, the different size fastener complicates maintenance and repair of the equipment because different tools are required, and correlating fasteners with the matching threads made more difficult. It is therefore desirable in some cases to be able to replace the original threads with threads of the same size so that the same size fastener may be utilized. In these cases, self-tapping inserts may be employed.
Self-tapping inserts are hardened steel cylinders, threaded on the exterior and, usually, in the interior. The interior thread diameter and pitch of the internal threads are those of the fastener to be installed. However, it is to be appreciated that there are times when it is desirable to replace a smooth bore which has been damaged for one reason or another with a new smooth bore. For example, in the stamping industry, a smooth hole utilized for a position pin may become damaged and need to be replaced.
The exterior of the self-tapping insert comprises a section which cuts new threads (the “cutting section”) and a section of threads which make up into the newly cut threads. The cutting section of the known self-tapping inserts is tapered and usually comprises three or more slots or holes, which interrupt the tapered threads, thereby forming teeth similar to those of a conventional thread tap. Typically a bolt (the “drive bolt”) is used to drive the self-tapping insert into a pilot hole in the base metal. This pilot hole is usually made by drilling out the damaged threads as described above to form a bore hole in the base metal. As the insert is turned, the teeth of the cutting section engage and remove the base metal until the insert is fully installed and flush with the exterior surface of the workpiece. The insert remains in place within the workpiece by an interference fit between the newly cut threads in the workpiece and the exterior threads on the insert.
The known self-tapping inserts have several disadvantages. Because the known self-tapping inserts are tapered on the tapping end (i.e., the end which is first inserted into the bore hole), the inserts have a tendency to start tapping crookedly. The person performing the tapping procedure has no simple way other than by visual inspection to ascertain whether the insert is entering the bore hole straight—i.e., whether the longitudinal axis of the insert coincides with the longitudinal axis of the bore hole. The only way to ensure that the prior-art insert enters the bore straight is to utilize a magnetic drill (“mag drill”) which attaches to the work-piece with an electromagnet. An example of such a mag drill is disclosed in U.S. Pat. No. 3,969,036 (Hougen). The procedure requires: (1) positioning the mag drill by means of a centering tool; (2) replacing the centering tool with a drill bit and drilling out the damaged threads; (3) customizing the drive bolt by removing its head so that it can be fitted to the chuck of the mag drill; (4) with the mag drill maintained in exactly the same location as established in step (1), threading the prior art insert onto the modified drive bolt and installing the modified drive bolt into the chuck of the mag drill; (5) driving the prior art insert two to three rotations with the mag drill, until it has started to cut new threads; and (6) completing the installation with a wrench, socket wrench, pneumatic impact wrench, mechanical torque multiplier, or hydraulic torque multiplier, depending upon the torque required to install the prior art insert.
It is important that the insert be installed straight, which means it must be correctly aligned at the initiation of the installation procedure. If the insert is too crooked during installation, the insert may shatter when partially installed because of the hardness of the insert. If the insert is installed crooked and does not shatter, the fastener will often not align correctly with the insert. The alignment problem becomes more severe for larger inserts. In recognition of this problem, one manufacturer of self-tapping inserts requires that the installation method for larger diameter inserts (such as larger than ¾ inch) include counter-boring or partially pre-tapping the pilot hole for the insert such that the insert will be properly aligned within the hole. Counter-boring or pre-tapping the pilot holes are demanding, time-consuming and expensive procedures requiring large-diameter drill bits and/or taps, often under difficult field conditions.
When a prior-art self-tapping insert is used to rethread a bore (or provide a threadless bore as the need may arise), the insert is driven in a clockwise rotation. As a consequence of this clockwise rotation, the exposed edge (hereinafter the “leading edge”) of an opening in the wall of the insert acts as a cutting edge for cutting the new threads as the leading edge cuts into the wall of the bored hole. The edge on the opposite side of the opening from the leading edge is hereinafter referred to as the “trailing edge”. On the known self-tapping inserts, the leading edge and trailing edge are of equal height. As a consequence of the equal height, metal chips created by the cutting of the new threads are forced into the new threads as the new threads are being created, causing binding and galling. The binding and galling require a very high torque and/or a combination of high torque and impact from heavy-duty pneumatic or hydraulic tools to overcome to properly seat the insert within the hole. As a consequence of the high torque requirement, another disadvantage of the known self-tapping inserts is manifested. The known self-tapping inserts cannot be through-hardened to a hardness of more than 52RC to 54RC without a risk of cracking, metal fatigue, etc. resulting from the application of the necessary torque and impact to properly seat the insert because of the galling and binding described above. This limitation on the hardness of known self-tapping inserts prevents use of the inserts in workpieces in which the base metal has a greater hardness, because the inserts are not sufficiently hard to cut threads in the base metal.
The known self-tapping inserts generally rely upon an interference fit between the newly cut threads and the external threads of the insert to prevent the insert from backing out of the base metal. The small metal chips generated by the cutting action of the insert assist the interference fit by wedging between the external threads of the insert and the new threads of the base metal. While this phenomenon is effective in preventing back-out of the insert from the base metal, it increases the torque requirements for installing the insert as described above.
The method of installing the known self-tapping inserts presents another disadvantage. The known self-tapping inserts are installed with a drive bolt having the same diameter and thread pitch as the insert. A nut and/or a combination of a nut and washers are utilized as a spacer between the head of the drive bolt and the insert. This spacer acts as a stop when the insert is inserted to the point that the top side of the insert is flush with the top surface of the workpiece. Because of the high torque levels required to install conventional self-tapping inserts, the drive bolt can seize up within the insert, particularly with the larger diameter inserts, causing the prior art insert to back out of the workpiece upon removal of the drive bolt. With this installation method, and because of the interaction of the chips with the exterior threads of the insert and the newly cut threads in the base metal, once this type of self-tapping insert is installed it cannot be removed. Moreover, with the drive bolt method of installation, the insert cannot be mounted such that the top side of the insert is placed below the top surface of the workpiece without creating custom tooling.
A need therefore exists for a self-tapping insert which satisfies one or more of the following criteria: (1) consistently remains aligned within the workpiece without the need for counter-boring and/or pre-tapping; (2) having a hardness in excess of 54RC yet capable of being installed without shattering; (3) a reduction in the necessary installation torque; and (4) having an installation method which prevents backing out of the insert from the workpiece because of seizing/galling with the drive bolt during the act of installation, or because the workpiece's installed fastener has itself seized with the insert.